Indoor air quality has an impact on the population's health and comfort. Indoor air pollution is a significant risk factor that may lead to multiple health problems, while people are increasingly spending time indoors. By applying the right air purification and/or humidification system the adverse effects of a poor indoor air quality can be minimized.
Indoor air pollutants are principally composed of three groups: particulate matter that include fine particles, tobacco smoke and radioactive particles; gases and vapors that include volatile organic (VOC) and inorganic compounds; and biological contaminants that include microorganisms such as fungi, spores, bacteria and viruses. Exposure to VOCs, in particular, may cause adverse health effects such as irritation of the eyes, skin, and respiratory tract and may also lead to more serious diseases including cancer and leukemia.
Common methods of controlling indoor air pollution and improving indoor air quality include controlling pollution sources, increasing the air exchange (ventilation) rate, and using air purifiers and air humidifiers. The use of air purifiers to eliminate indoor air pollutants is becoming increasingly popular. Traditional air purifiers use filters to remove particulate matters or use sorption materials (e.g., granular activated carbon) to adsorb polluting gases and/or odors from air.
Among the polluting gaseous compounds, the presence of formaldehyde gas has been identified as a particular risk. Formaldehyde is toxic, allergenic, and carcinogenic. At concentrations above 0.1 ppm in air, formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes. Inhaled formaldehyde at and above this concentration may cause headaches, a burning sensation in the throat, difficulty with breathing, and can trigger or aggravate asthma symptoms.
However, unlike other indoor volatile organic compounds that can be effectively removed by porous sorbent media via physical adsorption, formaldehyde cannot be satisfactorily removed by pure sorbent media due to its relatively low boiling point (−19.3° C. [−2.74° F.]) and thus high volatility. The VOC adsorption ability of activated carbon decreases for gases with decreasing boiling points: gases with boiling temperatures above room temperature can relatively easily be absorbed by activated carbon, while gases with boiling temperatures below room temperature are only moderately or poorly adsorbed.
To overcome the problems associated with the use of activated carbon, chemisorption filters may be employed. Such filters comprise specific chemical species that are impregnated in a porous substrate. The impregnated substrate may then be located as a chemisorption filter in an air purifier. The impregnated chemical species may react with formaldehyde. Chemical species capable of absorbing formaldehyde gas from air are principally organic amines. The reaction between the organic amine and formaldehyde is illustrated in Equation 1. After formaldehyde has reacted with the organic amine, it becomes a non-volatile compound that remains in the filter and is thereby removed from the air.HCHO+R—NH2→R—N═CH2+H2O  Equation 1The absorption performance of a chemisorption filter can be affected by the relative humidity (RH) of the air. In general, the filter's absorption capacity and one-pass absorption efficiency towards formaldehyde decreases with a decreasing RH, particularly so when the RH<30%. In dry air at RH<30%, the filter's air cleaning performance towards formaldehyde may therefore become unsatisfactory. It is therefore desirable to maintain the ambient RH at RH>30%. This is possible by wetting the chemisorption filter with water or with an aqueous chemical solution capable of absorbing formaldehyde from air.
The use of a fixed wetted filter in combination with an aqueous chemical solution for air cleaning is disclosed in WO2013128350A1/CN 104067060 A. The document describes a system including a fixed filter structure that is irrigated on its top with the chemical solution by means of a pump. When a polluting gas in air passes through the filter, the gas reacts with the chemical inside the filter structure and it is thereby removed from the air. The described fixed wetted filter can simultaneously act as a humidifying filter, which can be used to prevent the relative humidity (RH) of the ambient air from becoming too low to allow a satisfactory air cleaning performance towards the polluting gas. However, such a system requires a pump for enabling the filter wetting with the aqueous chemical solution, which can be noisy and costly, and may require periodic replacement, as well as periodic replacement of the filter, which is inconvenient and costly.